This invention relates generally to mechanical transfer systems and related devices for moving a selected article through a predetermined precision path of motion. More specifically, this invention relates to a relatively simple and relatively lightweight transfer mechanism for displacing a mobile carriage or the like at a relatively rapid speed and with a high degree of reliability and precision. The invention is particularly adapted for use in a video cassette storage and retrieval system wherein the mobile carriage is designed for transferring video cassettes back and forth between a cassette library and cassette playback equipment.
A wide variety of mechanical transfer systems are known in the art for displacing a variety of articles or devices through a preselected path of motion, including movement along or with respect to two or more orthogonal axes. For example, such transfer systems have been used in many industrial applications for moving manufactured articles from one processing station to another. Other types of transfer mechanisms have been used to support a manufacturing tool which is moved through a preprogrammed path for automatic or robotic performance of a manufacturing operation.
In general, however, previous transfer systems having bidirectional capability have experienced a number of disadvantages which have limited their general adoption and use. More specifically, in such transfer systems, inherent mechanical tolerance requirements have presented a significant obstacle to obtaining an output motion with a high degree of positional precision. As a result, when precise motion is desired, such transfer systems have included relatively costly bearing components while other systems have used relatively large and/or heavy structures for supporting moving components, for example, at widely spaced support points. However, these approaches undesirably increase the cost, complexity, size, and overall mass of the transfer system. Moreover, when precision movement is desired, the system components have been limited to relatively slow mechanical displacements due to the mass of the components and/or to avoid undesirable binding of close tolerance or heavy components.
More particularly, in accordance with traditional kinematics theory, mechanical devices constrained for precision movement along a specified axis or direction have required support at a minimum of three rigidly interconnected support points relative to a rigid frame. These support points define a triangular array lying within a plane which ordinarily corresponds with or extends in the direction of movement of the mechanical device. For example, in a mechanical device adapted for precision movement in an X-axis direction, at least three rigidly interconnected support points have been required wherein these support points lie within a common plane extending substantially in the X-axis direction. During X-axis movement of the mechanical device, the trio of support points cooperatively constrain the device against displacement in the Z-axis or Y-axis directions and further against pitch or yaw about the Z-axis or Y-axis. However, these requisite support points necessarily increase the X-axis span or size of the mechanical device with corresponding increases in overall mass, complexity, and cost. Attempts to reduce the X-axis spacing of the support points to reduce the size of the movable device have required expensive bearing structures and close mounting tolerances relative to the rigid frame.
By way of one specific example, video cassette storage and retrieval systems have been proposed for use by television broadcast stations for automated transfer of cassettes between a storage library and playback equipment, thereby permitting automated station operation with selected programs, commercial messages, and the like being played according to a timed, preprogrammed sequence. In such systems, a mobile carriage including a cassette pick-up and release unit is transported by a multidirectional transfer mechanism back and forth between the storage library and the playback equipment to select, transfer, play and return selected cassettes to the library. For proper operation without jamming or cassette misfeeds, the transfer mechanism must be capable of moving the carriage through precise displacements. However, as noted previously herein, precision movements have required relatively slow displacement speeds which are incompatible with the timing demands of a modern broadcast station wherein commercial message spots and the like may be as short as five seconds in duration thereby requiring relatively rapid cassette changeover capability. Moreover, to insure precision movements, previous transfer mechanisms have been relatively large in size to include the requisite mechanical support points, thereby rendering such mechanisms inconvenient for installation into a compact space efficient cassette storage and retrieval system.
There exists, therefore, a significant need for an improved transfer mechanism having bidirectional capability for use, for example, in rapid yet precision transfer of video cassettes and the like between a storage library and associated playback equipment. Moreover, there exists a need for such a transfer mechanism which is relatively simple, compact, and lightweight in construction. The present invention fulfills these needs and provides further related advantages.